Tapered ladder log periodic antenna



March 7, 1967 R. L. BELL 3,308,470

TAPERED LADbBR LOG PERIODIC ANTENNA Original Filed .Jan. 27, 196l 2 Sheets-Sheet 1 INVENTOR. ROSS L. BELL ATTORNEY March 7, 1967 I 2 R. L. BELL 3,308,470

TAPERED LADDER LOG PERIODIC ANTENNA Original Fild Jan. 27, 1961 2 Sheets-Sheet z m SW50; 0.]. BALLVIBU s.aMs/\ INVENTOR.

ROSS L. BELL ATTORNEY United States Patent Office 3,308,470 TAPERED LADDER LOG PERIODIC ANTENNA Ross Leon Bell, Dallas, Tex., assignor to Granger Associates, Palo Alto, Calif, a corporation of California Original application Jan. 27, 1961, Ser. No. 85,233, now Patent No. 3,134,979, dated May 26, 1964. Divided and this application Apr. 2, 1964, Ser. No. 356,751

4 Claims. (Cl. 343792.5)

This is a division of application Ser. No. 85,233, filed Jan. 27, 1961, now Patent No. 3,134,979.

This invention relates generally to antenna structures, and the invention relates more particularly to a novel symmetrical antenna of the log-periodic type.

Log-periodic antennae have been developed within the last few years which have the desirable properties of maintaining relatively constant radiation patterns and impedances over relatively large frequency bands of the order of :1 or even greater. While these antennae perform satisfactorily for many applications they are not compatible with systems operating in the high frequency range that require vertical polarization. In addition, the physical configuration of these log-periodic antennae as heretofore constructed leave much to be desired, both from the structural and fabrication points of view.

The early unidirectional log-periodic antennae usually consisted of two separate planar antennae that diverged from the apex or feedpoint with repetitious antenna elements whose dimensions increase with increasing distance from the apex of the array. These antennae occupied a considerable volume, thereby producing complex structural problems which are magnified when the antennae are to be operated in environments which have even moderate wind and icing conditions. In addition, the structures are asymmetrical, that it, the portion above the X-Z plane is not a mirror image of the portion below this plane, and this makes it impossible for imaging these antennae as over the ground or over a counterpoise. Thus, for vertical polarized applications these prior art antennae have been restricted to optimum performance only in a free space environment. Furthermore, the asymmetrical structure makes these antennae in use difiicult to maintain due to differential wind pressure on the two planar members or arms of the antenna,

More recently a balanced or symmetrical form of logperiodic antenna, known as the tapered ladder has been developed jointly by the applicant hereof and Arthur F. Wickersham, Jr., consisting of tapered plates of progressively larger dimension as measured from the apex and are shunt-excited as by leads. While the radiation patterns and impedances of the tapered ladder antenna are very satisfactory, in practice, however, its usefulness is limited to frequencies of relative short wavelengths due to the fact that the radiating elements are solid plates which render the tapered ladder extremely vulnerable to wind loading as well as extremely expensive at lower frequencies.

It is the principal object of the present invention to provide a novel antenna of the log-periodic type which is of simple construction and has highly desirable structural features, i.e., which is of relative light weight, rugged, and can withstand large wind loading, and is especially adapted for imaging over a ground or counterpoise, resulting in such case in a substantial reduction in the antenna height for the same range of operating frequency.

A feature of the present invention is to provide a novel antenna that is well adapted to be used as a folded monopole array operating in conjunction with the ground or counterpoise.

Another feature of the present invention is to provide ,a novel folded monopole array of the logarithmic type Patented Mar. 7, 1967 that comprises in effect a series of monopoles that diverge from an apex and in which the pertinent dimensions of the radiation increase uniformly and proportionally with the increase in distance from the apex of the structufei Another feature of the present invention is to provide a novel imaged broadband antenna employing rod or wire antenna elements in semi-trapezoidal form and which is capable of having an exceedingly wide frequency band where the monopole elements are shunt excited.

Another feature of the present invention is to provide an antenna of the above character wherein energy from a coaxial line feed, when acting as a transmitter, serves to excite the antenna by having one conduct-or connected to the ground system and its other conductor connected to alternate wires of the antenna through a feed line using couplers.

These and other features and advantages of the present invention will be more apparent after a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1a shows a sectional view through a prior art asymmetrical log-periodic antenna;

FIG. 1b is a perspective view which shows the antenna of FIG. 1a with the planar members 1 and 2 extending above and below the X-Z plane;

FIG. 2a is a perspective view which shows a tapered ladder antenna suitable for being mirrored or vertically imaged;

FIG. 2b is a perspective view which shows a tapered ladder antenna useful for free space environment;

FIG. 3a shows in elevation an imaged, folded monopole array embodying the present invention;

FIG. 3b is a perspective view of a portion of the structure shown in FIG. 3a;

FIG. 3c is a perspective fragmentary view of a monopole array employing a ground plane;

FIG. 4 illustrates a typical elevational pattern for the folded monoplane antenna of FIG. 3a;

FIG. 5 shows the VSWRs of the antenna structure of FIGS. 3a and 3b with respect to a nominal impedance of 50 ohms; and

FIG. 6 illustrates the geometry of the structure of FIG. 3a in plan.

Referring now to the drawingsQFIGS. 1a and lb show an early form of periodic antenna having two asymmetrical periodic plane members 1 and 2 that diverge from the apex or feedpoint 3. The said planar members being separated by an angle p as shown in FIG. 1a. Antennae of this nature occupy considerable volume, producing complex structural problems which are magnified when the antennae are operated in environments having even moderate wind and icing conditions, and since the antenna is asymmetrical, as shown in FIG. 1b, that is, the planar members 1 and 2 are not identical, it is impossible to image one-halfof this antenna above ground or a counter:-

poise. p

The structures shown in- FIGS. 2a and 2b are of the tapered ladder type, wherein tapered plates 5 are shunt excited by leads 6 or by a lead 6 and a counterpoise or ground plane 6. The radiation patterns and impedance structures of the antenna are quite satisfactory; however, in practice its usefulness is limited to short wavelengths due to cost of the plates used and to the fact that for the longer wavelengths the solid plates 5 offer too much wind resistance and are subject to ice loading. For ex- :ample, at 4 mc./s. this antenna would have solid plate radiating elements that are about feet high and 35 feet wide, which not only would be highly expensive but the solid plate construction produces enormous amounts of wind drag and ice loading even at designs of much shorter wavelength.

The novel antenna of this invention is symmetrical and can be used as a folded monopole array, resulting in a substantial reduction in size from that of a free space radiator. A preferred arrangement is shown in FIGS. 3a, 3b, and 3c, and consists of half of the folded dipole array shown in my co-pending application, Ser. No. 85,233. The antenna of FIG. 3a can be imaged. over either the ground or a counterpoise, a counterpoise 14 being shown in FIGS. 3a and 3b and a ground plane 23 in FIG. 3c. It will be noted that the folded monopol-es of FIG. 3a are carried by a catenary cable 16 suspended from a rear post support 19 through insulating U-shaped supports 17, as of Teflon or other insulating material, which are shown extending down from the cable in and under adjacent dipoles. Since the structure is imaged, its feed is unbalanced and is shown excited by means of a coaxial line with its outer conduutor 18 being connected to the ground system and its inner conductor 19 being coupled to wires 9 through a feed line 26 and capacity couplers 11 of the type disclosed in my above referred to parent case. Assuming a perfect ground system, the operation of this folded monopole array is similar to the folded dipole array in said parent case. This array produces a radiation pattern that is substantially constant with frequency and as shown in FIG. 4 produces a lobe that has a maximum radiation very near the ground and is highly directive.

In the geometry of this radiating structure as shown in FIG. 6, the angle a defines the extremities of equivalent folded dipole elements and their distances from the vertex of the structure may be designated as R. The subscripts nl, n, n+1, etc., denote particular radiating elements, while X denotes the half lengths of these elements. The geometry is such that the pertinent dimensions, R and X, pertaining to an individual element are the geometric mean of respective adjacent element dimensions, or

The parameter 6 in the above equations is a constant for a given antenna. This means that the folded dipole radiators differ from each other only by the scale factor, 6 The range of frequencies over which the structure will operate is determined by the size of the antenna, that is, by the number of radiators in the structure. Various combinations of design parameters will work satisfactorily, for example, with 6:0.90, R,,=O.90 R and X =0.90 X If an a of 40 is used, then the active portion of the antenna is approximately two-thirds of a wavelength from the apex or feedpoint of the antenna.

The radiation patterns and impedances of this novel antenna remain essentially constant over the operating frequency band. In FIG. 4 there is shown typical E and H plane patterns, the E pattern being shown in light line, and the H pattern being shown in heavy line.

In a typical instance, a folded monopole array, such as that shown in FIG. 3a, may have the following values: 6:0.875 and 06/2 is 20, where a is the vertical angle of the antenna. The distance R in the VHF model is of the order of 40 feet and the R in the HF model is 220 feet. An insulated matching capacitor shown consisting of a wire connected to the outer conductor 18 and insulated at 25' extends along an insulated portion of the feed line conductor 19 adjacent the apex of the structure to match the concentric line to the antenna. The folded monopoles 27 of this structure are shown secured at their lower ends to a central longeron 13, which longeron is conductively connected to the counterpoise 14.

In FIG. 30 a ground or ground plane 23 as of sheet metal is used as a counterpoise.

FIG. 5 shows the VSWRs of said monopole folded antenna structure with respect to a nominal impedance of 50 ohms. The VSWRs are less than 321 over the operating band.

Thus it will be seen that the novel monopole array of this invention is of extremely simple, rugged design and suitable for fabrication of large as well as small antennae, which is not true of the prior art. The array, being imaged over the ground, is easily supported and can be made readily resistant to wind pressures without undue whipping. The array is extremely simple and can be made in large sizes without undue wind pressures, which is not true of prior art antennae. Since this antenna can be fabricated from wires, fabrication costs are comparatively lower. It is evident that wind drag and icing conditions for this antenna are much less severe than for either the non-planar log-periodic or the tapered ladder antenna.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A broadband antenna comprising an antenna structure extending outwardly and upwardly from a vertex and having a plurality of mutually spaced, vertically disposed similar folded monopole radiators, each such radiator consisting of a pair of spaced parallel antenna wires with a connecting loop portion at their upper ends, said radiating antenna wires having their lower ends grounded, the said radiators being successively arranged in a plane such that the physical dimensions of the radiators and their spacings increase uniformly and proportionally with increase in distance from the vertex, a feed line, capacity coupling members interconnecting one antenna wire only of each of said folded radiators to one side of said feed line, the other side of said feed line being grounded such that, in use, alternating currents flow from said one side of said feed line through said coupling members around said folded radiators, and through ground to the other side of said line, whereby in operation a ground image of said antenna is established so that the output of the same is similar to that of a symmetrical folded dipole array.

2. An antenna as defined in claim 1 wherein said one side of said feed line is coupled by said coupling members to the radiating wire of each radiator that is more removed from the antenna vertex so that electrical energy traveling along said one side of the feed line toward the rear of said antenna passes through said coupling members to the rear longer wire of the active radiators whereby the energy in the shorter forward wire of a radiator lags in time phase from that in the rear longer wire, thereby yielding a resonant phase progression in the direction toward the vertex of the antenna causing radiation in that direction.

3. An antenna as defined in claim 2 wherein each folded radiator has its parallel spaced antenna wires extending orthogonally with respect to the bisector of the vertex angle of said antenna and its ground image, a single supporting post positioned at the rear of said antenna, a catenary cable connected at one end to said post and its other end to ground adjacent the vertex of said antenna, insulating members carried by said cable.

and connected to the loop portions of said radiators for supporting the latter, and an electrically conducting longeron extending along a line substantially at right angles to said antenna wires and leading from the rear of said antenna toward the antenna vertex, said longeron being electrically connected to said antenna wires and to the grounded side of said feed line.

4. An antenna as defined in claim 3 wherein an insulated conductor electrically connected to said longeron extends along said one side of said feed line for a distance adjacent the antenna vertex for matching the feed line to the antenna and wherein a counterpoise comprising a plurality of interlaced wires extends below said antenna, certain of said wires being electrically connected to said longeron.

References Cited by the Examiner UNITED STATES PATENTS Peterson 343886 Wickersham et a1. 343792.5 Wickersham 343792.5 Duhamel et a1. 343792.5 Wickersham et al. 343792.5 Bell 343792.5

10 ELI LIEBERMAN, Primary Examiner. 

1. A BROADBAND ANTENNA COMPRISING AN ANTENNA STRUCTURE EXTENDING OUTWARDLY AND UPWARDLY FROM A VERTEX AND HAVING A PLURALITY OF MUTUALLY SPACED, VERTICALLY DISPOSED SIMILAR FOLDED MONOPOLE RADIATORS, EACH SUCH RADIATOR CONSISTING OF A PAIR OF SPACED PARALLEL ANTENNA WIRES WITH A CONNECTING LOOP PORTION AT THEIR UPPER ENDS, SAID RADIATING ANTENNA WIRES HAVING THEIR LOWER ENDS GROUNDED, THE SAID RADIATORS BEING SUCCESSIVELY ARRANGED IN A PLANE SUCH THAT THE PHYSICAL DIMENSIONS OF THE RADIATORS AND THEIR SPACINGS INCREASE UNIFORMLY AND PROPORTIONALLY WITH INCREASE IN DISTANCE FROM THE VERTEX, A FEED LINE, CAPACITY COUPLING MEMBERS INTERCONNECTING ONE ANTENNA WIRE ONLY OF EACH OF SAID FOLDED RADIATORS TO ONE SIDE OF SAID FEED LINE, THE OTHER SIDE OF SAID FEED LINE BEING GROUNDED SUCH THAT, IN USE, ALTERNATING CURRENTS FLOW FROM SAID ONE SIDE OF SAID 